We have completed measurements of the theoretical stoichiometry of H+ translocation coupled to electron transport by tumor cell mitochondria. Nearly 12 H+ are translocated per pair of electrons transferred from NADH to O2, exactly as in mitochondria from normal tissues. Tumor mitochondria therefore possess the enzymatic equipment to manufacture ATP as efficiently as normal cells. However, with this baseline information we are now in a much better position to determine why oxidative phosphorylation in intact, living tumor cells is qualitatively and quantitatively different than in normal cells. This past year more effort was placed on the anomalous pathway of pyruvate oxidation in mitochondria of tumor cells. Whereas pyruvate, the product of glucose breakdown, is normally oxidized in acetyl-CoA and then to citrate in mitochondria from all normal tissues, we have made the surprising observation that a large fraction of the pyruvate added to aerobic Ehrlich tumor mitochondria was undergoing nonoxidative decarboxylation to yield CO2 and some other product not normally participating in the tricarboxylic acid cycle. Quantitative analysis by both gas-\and high-performance liquid chromatography has revealed that the other product is the four-carbon compound acetoin, which is not a normal metabolite in animal metabolism, although it is formed by yeast and many bacteria. Tumor mitochondria were also found to oxidize acetoin quite readily to as yet unknown products, whereas normal liver mitochondria are totally unable to oxidize this unusual metabolite. These observations indicate that there is a functional defect in the pyruvate dehydrogenase complex of tumor mitochondria so that acetoin is formed instead of the normal acetyl-CoA. Efforts are under way to determine the fate and possible function of acetoin in the blood of tumor-bearing animals. (E)